Conductive pastes are used for forming wiring on electronic components such as a chip resistor, a chip capacitor, and a solar battery, as well as on electronic package components such as a printed wiring board and a substrate having a through hole. Conductive pastes can be also used for electrode or wiring to be connected to a transistor for controlling pixel switching of a display. For most of the existing conductive pastes, a silver paste that is excellent in oxidation resistance is used. However, silver is expensive and, in addition, has a problem that migration failure easily occurs in fine-pitched wiring. Therefore, studies have been made for obtaining wiring structures with low resistance and high reliability by producing a copper paste using copper in place of silver and then firing the copper paste.
When a silver paste is fired in the air, a binder resin reacts with the air, and thereby the amount of resin that remains in the wiring after firing can be decreased as much as possible, and increase in wiring resistance due to remaining of resin can be suppressed. On the contrary, since copper is easily oxidized in atmosphere containing oxygen, the conductive copper paste needs to be fired in an inert gas or in a vacuum. In this case, there has been a problem that a binder resin component remains in a wiring due to oxygen deficiency, thus deteriorating the sinterability, resulting in increasing the wiring resistance. Furthermore, even when the copper paste is fired in an inert gas or in a vacuum, it has been difficult to properly suppress oxidation of copper particles, so that copper particles are oxidized at the time of firing and the wiring resistance tends to be increased.
As a method using inexpensive conductive paste of copper particles in place of expensive silver particles, for example, Patent Document 1 discloses a method for sintering while heating at not lower than a temperature at which resin is decomposed and disappears in the air, and reducing copper oxide powders produced at this time into copper. Patent Document 2 discloses a method for forming a sintered body by carrying out a de-binder step in a non-oxidizing atmosphere, and lowering the temperature in an oxidizing atmosphere to oxidize copper, and further reducing the copper. Patent Document 3 discloses a method in which the oxidation treatment is carried out at a temperature exceeding an oxidation start temperature, and the reduction treatment is carried out at a temperature not lower than the reduction start temperature. Patent Document 4 discloses a method for drying at a temperature less than 150° C., followed by pressurizing, heating, and carrying out reduction treatment.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H07-45931
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2002-43747
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2009-231755
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2012-204466